This invention relates to a method of coating zinc metal and/or zinc coated articles, such as galvanized steel, with a permanganate composition to provide the zinc with greater paint adhesion and to the zinc articles so coated.
Unpassified zinc generally will form a thin film of zinc oxide which will prevent the adhesion of paint. Passification will prevent the growth of zinc oxides ( see British patent No. 592,072; Wendorff Z., Zolnierowicz, A.; Ochronaprzad Korozja, 13, 1 (1970); Ostrander, G. W.: Plating, 38 1033 (1951); and British Patent No. 594,699). Typical passification processes use a dichromate or a chromate composition. The compositions are applied by simple immersion or by electrochemical methods (see Fishlock, D. J.: Product Finishing, 12, 87 ( 1959). A number of different PH""s, immersion times and temperatures may be used. The use of a chromate or dichromate passification will generally increase the corrosion resistance. Corrosion resistance is generally tested by a saltxe2x80x94spray (xe2x80x9cASTMxe2x80x94B117xe2x80x9d testing specification) see: Stareck, J. E., Cybulskis, W. S.: Proc. Am. Electroplaters Soc. 34, 235 (1947). The hexavalent chromium present in the chromate and/or dichromate compositions is extremely toxic, and as such, is being banned from use in many European countries and many areas of the United States.
My patent U.S. Pat. No. 5,8200,741 provides for the passification of zinc using a trivalent chromium composition. My present invention provides for the passification of zinc with a chromium-free permanganate composition. The chromium coated zinc and/or zinc coated articles must generally be painted prior to the chromium completely setting. If it is not, the paint will not adequately adhere to the chromium surface.
The present invention concerns a method for coating zinc and/or zinc coated articles, i.e. zinc plated steel, with a nonxe2x80x94hexavalent chromium permanganate coating composition having a PH of about 2.0 to 9.0.
The invention also concerns a passified zinc or zinc coated article having coated thereon a manganese oxide protective coating produced by an alkali metal permanganate solution. The alkali metal is selected from potassium, sodium or lithium. The preferred alkali metal being potassium. The concentration of permanganate necessary to produce an acceptable coating being a minimum of 0.001 moles per liter. With potassium permanganate this corresponds to about 0.16 grams per liter. The maximum concentration of the permanganate is the saturation point of the salt being used. The solution may have a temperature ranging from above the freezing point of the solution to its boiling point. The preferred temperature range being 60xc2x0 F. to 180xc2x0 F. As the temperature of the solution rises, less immersion time is required to form a corrosion resistant coating on the surface of the zinc. The immersion time for preparing a corrosion resistant coating on a zinc surface is about 45 seconds to 3 minutes at 60xc2x0 F. Preferred immersion time is 45-90 seconds. A longer immersion time then the predetermined optimum time will not increase the performance level of the coating to any great extent.
Other compounds may be added to the composition, such as PH buffers, to adjust the PH of the composition. The compounds used should not have an adverse effect upon the corrosion resistant or paint adhesion properties of the coating.
A PH below 2 and above 9 would have a deleterious affect on the zinc.
A desirable protective coating is one which will allow the zinc surface to be painted and pass the dry paint adhesion according AAMA coating specification: 605.2-85, which is incorporated herein and submitted with this application.
The following examples illustrate the invention in detail, and are not intended to limit the scope of the invention. In the examples, the zinc surface of a zinc plated steel or a pure zinc sheet was cleaned of oils and loose dirt with a nonxe2x80x94ionic detergent. The non-ionic cleaned zinc surface was further cleaned by being made the cathode of an electrolytic cell of 12 volts and 10 amps for 30 seconds in a dilute sodium carbonate solution (2.5 grams per liter) to obtain a chemically clean and receptive surface. The surface was then washed with one percent nitric acid to remove all alkali residue and rinsed with D. I. water. This cleaned pure zinc sheet and/or zinc plated steel sheet was passified immediately. The zinc sheet and/or zinc plated sheet used in the examples were metal strips three inch by five inch and {fraction (1/16)} inch thick. The zinc plated steel sheet had a zinc coating thickness of 0.0002 inches.
When subjected to a five percent neutral saltxe2x80x94spray (PH=6.5 to 7.2) according to xe2x80x9cASTM Standard Bxe2x80x94117xe2x80x9d untreated, but cleaned as specified above, pure zinc and zinc plated steel will show signs of corrosion, as evidenced by a white film of zinc oxides and hydroxides. Zinc plated steel will show signs of corrosion after one-half to one hour of exposure. Pure zinc will show signs of corrosion after one to two hours of exposure.
In all of the examples below, unless otherwise indicated, one panel was treated in order to test for paint adhesion after being allowed to dry by sitting in the open air for 24 hours. This panel was painted with a flat black enamel from xe2x80x9cRustxe2x80x94Oleumxe2x80x9d Corp., Product No. 7776, allowed to dry for seven days and tested for dry paint adhesion according to xe2x80x9cAAMAxe2x80x9d coatings specification: 605.2-85.
Example 2 is given to show that a chromate based coating will not give acceptable paint adhesion when allowed to age for 24 hours. Examples 5, 8, 10, 15, 17, and 19 illustrate the paint adhesion failure of the permanganate based coating when used outside established PH and concentration limits.